The invention relates to a device for optically scanning tracks in a radiation-reflecting information plane, which device comprises a radiation source for supplying a scanning beam, an objective system for focusing the scanning beam to a scanning spot on the information plane, a dividing element having two sub-areas located symmetrically at both sides of a dividing line and arranged in the optical path of radiation from the information plane for splitting at least a part of the radiation into two scanning sub-beams, and a radiation-sensitive detection system. (The above-mentioned symmetry is understood to mean that the surfaces of the sub-areas are located symmetrically at both sides of the dividing line.)
A device of this type, which in principle is suitable for reading and writing information from and in, respectively, an optical record carrier, is known from U.S. Pat. No. 4,665,310. The dividing element in this device is a diffraction element i.e., grating having two sub-gratings at both sides of a dividing line. The diffraction element fulfills two functions. In the first place, the grating ensures that a part of the radiation reflected by the information plane is deflected from the path of the radiation emitted by the radiation source so that a detection system can be placed in the path of the reflected radiation. In the second place, the grating splits the reflected scanning beam into two scanning sub-beams which are required for generating a focus error signal in accordance with the Foucault method, i.e., a signal which is proportional to a deviation between the focusing plane of the scanning beam and the information plane.
In this device not only a focus error signal but also a tracking error signal must be generated. A tracking error is a deviation between the center of the scanning spot and the centerline of the track to be scanned. In the device described in U.S. Pat. No. 4,665,310, a tracking error is detected by means of the same detection systems with which a focus error is also detected.
To this end, the diffraction element in the known device is oriented in such a way that the dividing line between the sub-gratings is parallel to the effective track direction, i.e., the projection on the diffraction element of the line tangent to the tracks at the location of the scanning spot. The quantity of radiation in each scanning sub-beam deflected by the sub-gratings is determined by means of two detectors arranged in one of these sub-beams. The tracking error signal is obtained by determining, for each detector, a signal which is proportional to the incident radiation and by subtracting these signals from each other. This way of generating the tracking error signal is also referred to as the push-pull method.
A drawback of the known device is the radial to focus crosstalk. This is the phenomenon of the focus error signal being dependent on the transversal position of the scanning spot with respect to the tracks, also referred to as the radial position in the case of a disc-shaped record carrier. The crosstalk gives rise to a deviation between the information plane and the focusing plane of the scanning spot, which is set by the control of the objective lens. The magnitude of this focus offset is dependent on the transversal position of the scanning spot with respect to the track to be scanned. The crosstalk further influences the ratio between the magnitude of the focus error signal and the magnitude of the focus error. Dependent on the transversal position of the scanning spot, the loop gain in the focus control loop may vary by a factor of more than four. The crosstalk problems become most clearly manifest when the scanning spot is moved transversally across the tracks, for example when a track to be scanned is being searched. The transversal position of the scanning spot is then controlled by means of information read from the information plane during searching, for example, address information or the number of passed tracks. During searching the offset causes an oscillation in the axial position of the scanning spot, hence a periodically changing defocusing of the scanning spot so that the information in the information plane is not read satisfactorily and consequently the searching process is disturbed. The loop gain changes may lead to a complete loss of focus so that the device must be restarted.